lec microwave

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Microwave Communication

© 2008 The McGraw-Hill Companies

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Microwave Concepts

Microwaves are the ultrahigh, superhigh, and extremely high frequencies directly above the lower frequency ranges where most radio communication now takes place and below the optical frequencies that cover infrared, visible, and ultraviolet light.


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Microwave Concepts

Microwave Frequencies and Bands The practical microwave region is generally considered

to extend from 1 to 30 GHz, although frequencies could include up to 300 GHz.

Microwave signals in the 1- to 30-GHz have wavelengths of 30 cm to 1 cm.

The microwave frequency spectrum is divided up into groups of frequencies, or bands.

Frequencies above 40 GHz are referred to as millimeter (mm) waves and those above 300 GHz are in the submillimeter band.


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Microwave Concepts

: Microwave frequency bands.


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Microwave Concepts

Benefits of Microwaves Moving into higher frequency ranges has helped to

solve the problem of spectrum crowding. Today, most new communication services are assigned

to the microwave region. At higher frequencies there is a greater bandwidth

available for the transmission of information. Wide bandwidths make it possible to use various

multiplexing techniques to transmit more information. Transmission of high-speed binary information requires

wide bandwidths and these are easily transmitted on microwave frequencies.


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Microwave Concepts

Disadvantages of Microwaves The higher the frequency, the more difficult it becomes

to analyze electronic circuits. At microwave frequencies, conventional components

become difficult to implement. Microwave signals, like light waves, travel in perfectly

straight lines. Therefore, communication distance is limited to line-of-sight range.

Microwave signals penetrate the ionosphere, so multiple-hop communication is not possible.


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Microwave Concepts

Microwave Communication Systems: Transmission Lines Coaxial cable, most commonly used in lower-frequency

communication has very high attenuation at microwave frequencies and conventional cable is unsuitable for carrying microwave signals.

Special microwave coaxial cable that can be used on bands L, S, and C is made of hard tubing. This low-loss coaxial cable is known as hard line cable.

At higher microwave frequencies, a special hollow rectangular or circular pipe called waveguide is used for the transmission line.


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Microwave Lines and Devices

Although vacuum and microwave tubes like the klystron and magnetron are still used, most microwave systems use transistor amplifiers.

Special geometries are used to make bipolar transistors that provide voltage and power gain at frequencies up to 10 GHz.

Microwave FET transistors have also been created. Monolithic microwave integrated circuits (MMICs) are

widely used.


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Microwave Transistors The primary differences between standard lower-

frequency transistors and microwave types are internal geometry and packaging.

To reduce internal inductances and capacitances of transistor elements, special chip configurations known as geometries are used.

Geometries permit the transistor to operate at higher power levels and at the same time minimize distributed and stray inductances and capacitances.


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Microwave Transistors The GaAs MESFET metal semiconductor field effect

transistor, a type of JFET using a Schottky barrier junction, can operate at frequencies above 5 GHz.

A high electron mobility transistor (HEMT) is a variant of the MESFET and extends the range beyond 20 GHz by adding an extra layer of semiconductor material such as AlGaAs.

A popular device known as a heterojunction bipolar transistor (HBT) is making even higher-frequency amplification possible in discrete form and in integrated circuits.


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Microwave transistors. (a) and (b) Low-power small signal. (c) FET power. (d) NPN bipolar power.


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Small-Signal Amplifiers: Transistor Amplifiers A low-noise transistor with a gain of about 10 to 25 dB is

typically used as a microwave amplifier. Most microwave amplifiers are designed to have input

and output impedances of 50 Ω. The transistor is biased into the linear region for class A

operation. RFCs are used in the supply leads to keep the RF out

of the supply and to prevent feedback paths that can cause oscillation and instability in multistage circuits.

Ferrite beads (FB) are used in the collector supply lead for further decoupling.


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Small-Signal Amplifiers: MMIC Amplifiers A common monolithic microwave integrated circuit

(MMIC) amplifier is one that incorporates two or more stages of FET or bipolar transistors made on a common chip to form a multistage amplifier.

The chip also incorporates resistors for biasing and small bypass capacitors.

Physically, these devices look like transistors.


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Small-Signal Amplifiers: Power Amplifiers A typical class A microwave power amplifier is designed

with microstrip lines used for impedance matching and tuning.

Input and output impedances are 50 Ω. Typical power-supply voltages are 12, 24, and 28 volts. Most power amplifiers obtain their bias from constant-

current sources. A single-stage FET power amplifier can achieve a

power output of 100 W in the high UHF and low microwave region.


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Waveguides

Waveguides Most microwave energy transmission above 6 GHz is

handled by waveguides. Waveguides are hollow metal conducting pipes

designed to carry and constrain the electromagnetic waves of a microwave signal.

Most waveguides are rectangular. Waveguides are made from copper, aluminum or brass. Often the insides of waveguides are plated with silver to

reduce resistance and transmission losses.


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Waveguides

Wave paths in a waveguide at various frequencies. (a) High

frequency. (b) Medium

frequency. (c) Low

frequency. (d) Cutoff

frequency.


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Waveguides

Waveguide Hardware and Accessories Waveguides have a variety of special parts, such as

couplers, turns, joints, rotary connections, and terminations.

Most waveguides and their fittings are precision-made so that the dimensions match perfectly.

A choke joint is used to connect two sections of waveguide. It consists of two flanges connected to the waveguide at the center.

A T section or T junction is used to split or combine two or more sources of microwave power.


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Waveguides

A choke joint permits sections of waveguide to be interconnected withminimum loss and radiation.


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Microwave Semiconductor Diodes

Small Signal Diodes Diodes used for signal detection and mixing are the

most common microwave semiconductor devices. Two types of widely used microwave diodes are:

Point-contact diode Schottky barrier or hot-carrier diode


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Small Signal Diodes: Point-Contact Diode The oldest microwave semiconductor device is the point-

contact diode, also called a crystal diode. A point-contact diode is a piece of semiconductor material

and a fine wire that makes contact with the semiconductor material.

Point-contact diodes are ideal for small-signal applications.

They are widely used in microwave mixers and detectors and in microwave power measurement equipment.


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Small Signal Diodes: Hot Carrier Diodes For the most part, point-contact diodes have been

replaced by Schottky diodes, sometimes referred to as hot carrier diodes.

Like the point-contact diode, the Schottky diode is extremely small and has a tiny junction capacitance.

Schottky diodes are widely used in balanced modulators and mixers.

They are also used as fast switches at microwave frequencies.


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Hot carrier or Schottky diode.


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Oscillator Diodes Three types of diodes other than the tunnel diode that

can oscillate due to negative resistance characteristics are: Gunn diode IMPATT diode TRAPATT diode


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Oscillator Diodes: Gunn Diodes Gunn diodes, also called transferred-electron

devices (TEDs), are not diodes in the usual sense because they do not have junctions.

A Gunn diode is a thin piece of N-type gallium arsenide (GaAs) or indium phosphide (InP) semiconductor which forms a special resistor when voltage is applied to it.

The Gunn diode exhibits a negative-resistance characteristic.

Gunn diodes oscillate at frequencies up to 150 GHz.


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Oscillator Diodes: IMPATT and TRAPATT Diodes Two microwave diodes widely used as oscillators are

the IMPATT and TRAPATT diodes. Both are PN-junction diodes made of silicon, GaAs, or

InP. They are designed to operate with a high reverse bias

that causes them to avalanche or break down. IMPATT diodes are available with power ratings up to

25 W to frequencies as high as 300 GHz. IMPATT are preferred over Gunn diodes if higher power

is required.


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PIN Diodes A PIN diode is a special PN-junction diode with an I

(intrinsic) layer between the P and the N sections. The P and N layers are usually silicon, although GaAs

is sometimes used and the I layer is a very lightly doped N-type semiconductor.

PIN diodes are used as switches in microwave circuits. PIN diodes are widely used to switch sections of

quarter- or half-wavelength transmission lines to provide varying phase shifts in a circuit.


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Microwave Antennas

Horn Antenna Microwave antennas must be some extension of or

compatible with a waveguide. Waveguide are not good radiators because they

provide a poor impedance match with free space. This results in standing waves and reflected power.

This mismatch can be offset by flaring the end of the waveguide to create a horn antenna.

Horn antennas have excellent gain and directivity. The gain and directivity of a horn are a direct function of

its dimensions; the most important dimensions are length, aperture area, and flare angle.


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Microwave Antennas

Basic horn antenna.


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Microwave Antennas

Parabolic Antennas A parabolic reflector is a large dish-shaped structure

made of metal or screen mesh. The energy radiated by the horn is pointed at the

reflector, which focuses the radiated energy into a narrow beam and reflects it toward its destination.

Beam widths of only a few degrees are typical with parabolic reflectors.

Narrow beam widths also represent extremely high gains.


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Microwave Antennas

Cross-sectional view of a parabolic dish antenna.


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Microwave Antennas

Parabolic Antennas: Feed Methods A popular method of feeding a parabolic antenna is an

arrangement known as a Cassegrain feed. The horn antenna is positioned at the center of the

parabolic reflector. At the focal point is another small reflector with either a

parabolic or a hyperbolic shape. The electromagnetic radiation from the horn strikes the

small reflector, which then reflects the energy toward the large dish which radiates the signal in parallel beams.


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Microwave Antennas

Cassegrain feed.


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Microwave Antennas

Helical Antennas A helical antenna, as its name suggests, is a wire helix. A center insulating support is used to hold heavy wire or

tubing formed into a circular coil or helix. The diameter of the helix is typically one-third

wavelength, and the spacing between turns is approximately one-quarter wavelength.

The gain of a helical antenna is typically in the 12- to 20-dB range and beam widths vary from approximately 12° to 45°.

Helical antennas are favored in many applications because of their simplicity and low cost.


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Microwave Antennas

The helical antenna.


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Microwave Antennas

Bicone Antennas One of the most widely used omnidirectional microwave

antennas is the bicone. The signals are fed into bicone antennas through a

circular waveguide ending in a flared cone. The upper cone acts as a reflector, causing the signal to

be radiated equally in all directions with a very narrow vertical beam width.


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Microwave Antennas

The omnidirectional bicone antenna.


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Microwave Antennas

Dielectric (Lens) Antennas Dielectric or lens antennas use a special dielectric

material to collimate or focus the microwaves from a source into a narrow beam.

Lens antennas are usually made of polystyrene or some other plastic, although other types of dielectric can be used.

Their main use is in the millimeter range above 40 GHz.


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Microwave Antennas

Lens antenna operations. (a) Dielectric lens. (b) Zoned lens.


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Microwave Antennas

Patch Antennas Patch antennas are made with microstrip on PCBs. The antenna is a circular or rectangular area of copper

separated from the ground plane on the bottom of the board by the PCB’s insulating material.

Patch antennas are small, inexpensive, and easy to construct.


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Microwave Antennas

Intelligent Antenna Technology Intelligent antennas or smart antennas are antennas

that work in conjunction with electronic decision-making circuits to modify antenna performance to fit changing situations.

They adapt to the signals being received and the environment in which they transmit.


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TV Smart Antenna Multi-Directional HDTV

Multiple–radio smart antenna platform

the Smart BRO antenna.


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Microwave Antennas

Intelligent Antenna Technology Also called adaptive antennas, these new designs

greatly improve transmission and reception in multipath environments and can also multiply the number of users of a wireless system.

Some popular adaptive antennas today use diversity, multiple-input multiple-output, and automatic beam forming.


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Microwave Antennas

Adaptive Beam Forming Adaptive antennas are systems that automatically

adjust their characteristics to the environment. They use beam-forming and beam-pointing techniques

to zero in on signals to be received and to ensure transmission under noisy conditions.

Beam-forming antennas use multiple antennas such as phase arrays.


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Microwave Antennas

Adaptive Beam Forming There are two kinds of adaptive antennas: switched

beam arrays and adaptive arrays. Both switched beam arrays and adaptive arrays are

being employed in some cell phone systems and in newer wireless LANs.

They are particularly beneficial to cell phone systems because they can boost the system capacity.


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Microwave Applications

Major applications of microwave radio.


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Radar The electronic communication system known as radar

(radio detection and ranging) is based on the principle that high-frequency RF signals are reflected by conductive targets.

In a radar system, a signal is transmitted toward the target and the reflected signal is picked up by a receiver in the radar unit.

The radar unit can determine the distance to a target (range), its direction (azimuth), and in some cases, its elevation (distance above the horizon).


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16-7: Microwave Applications

Radar There are two basic types of radar systems: pulsed and

continuous-wave (CW). The pulsed type is the most commonly used radar

system. Signals are transmitted in short bursts or pulses. The time between transmitted pulses is known as the

pulse repetition time (PRT). In continuous-wave (CW) radar, a constant-amplitude

continuous microwave sine wave is transmitted.


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Radar: UWB The newest form of radar is called ultrawideband

(UWB) radar. It is a form of pulsed radar that radiates a stream of very

short pulses several hundred picoseconds long. The very narrow pulses give this radar extreme

precision and resolution of small objects and details. The low power used restricts operation to short

distances.

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